I have a Trek FX 7.1 that I've rode for several years now and replaced parts as they've worn out.

Now I'd like to upgrade the whole drive train with Tiagra 2x10 flat bar shifters, derailleurs, crank, cassette, wheels, etc.

So, my question is, which bottom bracket do I need to replace the old one? There are choices of english thread or italian thread and 68mm or 70mm. Additionally, if there is a higher quality one someone would recommend over the taigra one, that would be appreciated. Please specify size and thread needed.

  • 1
    If you're doing that you'd be better off just buying a 7.5 fx or whatever in terms of money.
    – Batman
    Feb 25, 2015 at 21:20
  • Hmmm, that bike is still in production but they don't mention the bb on their spec sheet. For sure it will be English - Italian threading is basically confined to Italian high-end road bikes. You might need to measure the bb that is in there to get the exact size. Shimano offer several groupsets at different levels of quality, check their web site (Tiagra is middle of the road).
    – PeteH
    Feb 25, 2015 at 21:23
  • 2
    I could not agree more with Batman. This project is really not going to be worth the financial investment. The 7.5FX will have the parts and wheels you want with a lighter frame, carbon fork, and many other niceties (ergonomically shaped grips, probably better brakes, probably lighter seatpost, handlebar, etc).
    – pjd
    Feb 25, 2015 at 23:26
  • 1
    At this point, you're basically just keeping the frame, saddle, seatpost, pedals and bars, which isn't worth it on a bike which is like 400-450 new.
    – Batman
    Feb 25, 2015 at 23:39

2 Answers 2


Assuming that you own the 2012 model, your bottom bracket should be a Shimano M171, according to the Trek Archives.

Found a Shimano Technical document that says you should have a 68mm thread width.


I found this question by chance, it is great that Stack Exchange has also a bike group! In my case (Trek 7.1 from 2015, Shimano M131 crankset) it seems that the bottom bracket is a Shimano BB-UN26, 68 mm diameter as posted above. But there are different axis lengths, 110/113/117.5/122 mm - anyone knows the right one?

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